A semiconductor device has a deep trench at least 10 microns deep, with a silicon dioxide liner on sidewalls and bottom of the deep trench. It is desirable to remove the silicon dioxide liner at the bottom of the trench without substantially reducing the silicon dioxide liner on the sidewalls of the deep trench, in order to make a contact to the substrate. A reactive ion etch (RIE) process used to remove the silicon dioxide liner at the bottom of the trench to make contact to the substrate has high ion energies which also remove dielectric material from the liner at the top of the deep trench, which undesirably widens a top portion of the deep trench. Widening the top portion necessitates a thicker layer of deposited silicon dioxide in the liner, which disadvantageously increases fabrication cost.